Due to the high prices of oils and regulation of exhaust gases throughout the world, environment-friendly policies and improvement of fuel efficiency have become core goals in the development of vehicles. Accordingly, the vehicle manufacturers are making a lot of efforts to develop technologies for reducing use of fuels and generation of exhaust gases for the environment-friendly policies and high fuel efficiency.
Under the background, many interests and efforts are being made in the development of technologies such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), which combine the power of an engine and a motor for high fuel efficiency.
The disclosure of this section is to provide background of the invention. Applicant notes that this section may contain information available before this application. However, by providing this section, Applicant does not admit that any information contained in this section constitutes prior art.